feat: add mmd-it marketplace + mmd-workspace plugin (4 MMD skills)

Org-wide Claude Desktop plugin marketplace for MMD Group IT. Adds the
mmd-it marketplace manifest and the mmd-workspace plugin
(installationPreference: required) carrying four skills grounded in
real MMD material: mmd-document-style, mmd-email-style,
mmd-it-support, mmd-data-handling. Public repo, no secrets — MCP
credentials are delivered separately via managedMcpServers.

plugins/mmd-workspace/skills/mmd-email-style/SKILL.md

@@ -0,0 +1,52 @@
+---
+name: mmd-email-style
+description: Use when drafting or sending any email on behalf of MMD or an MMD Group IT/Support matter — acknowledgements, clarification questions, progress updates, completion messages, or anything referencing an MMD Support (GLPI) ticket. Triggers on "draft an email", "reply to this ticket", "MMD #", "helpdesk", "email the requester", or "write to the user about their request".
+---
+
+# MMD email conventions
+
+MMD email is clean plain text, not marketing HTML, and it carries a specific ticket-reference
+and copy convention tied to MMD Support (the GLPI system at `support.baobab-ts.com`).
+
+## Format
+
+- Clean, well-spaced **plain text**. No HTML templates, no colored boxes, no signature images.
+- Short paragraphs, real line breaks between them — not one dense block of prose.
+- No filler ("I hope this finds you well", "just circling back") and no over-explaining. State
+  what happened and what, if anything, the reader needs to do.
+
+## The `[MMD #ticket]` line
+
+When an email is about an MMD Support ticket, put the ticket reference on its own first line,
+in the exact form `[MMD #<ticket number>]` — e.g. `[MMD #368]`. This is the same marker MMD
+Support's own inbound mail bridge looks for when it turns email into a ticket (it polls the
+support inbox and creates/updates a ticket from a message carrying this marker), so using the
+real format on outbound mail keeps the thread matched to the right ticket instead of spawning
+a duplicate one.
+
+## Copy MMD Support on the loop
+
+Acknowledgements, clarification questions, progress updates, and completion messages that
+relate to an MMD Support matter should copy `helpdesk@Fabrimetal.net` — that address is the
+outbound side of the same ticket system (GLPI relays through it), so copying it keeps the
+ticket's own record in sync with what the requester was actually told. Skip this only when the
+sender has an explicit reason not to for that specific thread.
+
+After sending a batch of these, it is worth a second check that MMD Support did not spawn a
+duplicate ticket from your own outbound copy landing back in the inbox — the mail bridge polls
+every few minutes, so give it one cycle before assuming the thread is clean.
+
+## Signing
+
+Sign as whoever is actually writing. Do not adopt another identity's signature. If you are an
+AI assistant sending mail on behalf of a specific person, make that relationship explicit in
+the signature (name the assistant and who it is acting for) rather than signing as if you were
+that person, and rather than leaving the email unsigned.
+
+## What NOT to do
+
+- Do not paste a ticket number into the subject line only and skip the `[MMD #...]` body line —
+  the mail bridge keys off the body marker.
+- Do not send an MMD Support–related email without helpdesk on copy unless there is a stated
+  reason for that thread.
+- Do not write long, hedged emails. State the fact, state the ask (if any), stop.